Hexavalent chromium (Cr6+) has been used in the surface passivation film of galvanized steel or aluminum alloy for a long time. According to the EU environmental directives of ROHS (The Restriction of Hazardous Substances in Electrical and Electronic Equipment) and WEEE (Waste Electronics and Electrical Equipment), United States, China, Japan and other countries started to limit the use of Cr6+ in 2006 as well. So far, the popular alternative method is using organic-inorganic hybrid resins (O/I hybrid resins). However, the conventional O/I hybrid resins need a large amount of organic resin to maintain the film-forming ability, and then result in the insufficient weather resistance and heat resistance.
In addition, the passivation film requires not only good corrosion resistance, but also sufficient conductivity and heat resistance, due to the concern of anti-static or high temperature application. Besides, most of galvanized steels are currently used as casing materials in electronic products, such as refrigerators, ovens, computers, and washing machines. The requirement of environmental-friendly passivation treatment is indeed increasing.